Helicoidal positive displacement pumps, sometimes referred to as Moineau-type pumps, have a wide variety of applications, including the oil producing and food processing industry, where they are used to pump fluids containing solids. In addition, helicoidal motors, which are essentially helicoidal pumps operating in reverse, are used widely in the oil drilling industry. In this application, the drilling mud is used as the driving fluid for a helicoidal motor that serves to rotate the drill bit.
Typically, a helicoidal pump/motor is comprised of a stationary stator and a helical rotor that orbits eccentrically as it rotates within the stator. The rotor is typically metallic and has one or more helical lobes spiraling around its outside diameter. The stator has a number of helical lobes that form grooves in the stator inner surface that spiral along its length, with the number of helical lobes in the rotor being one less than the number of helical grooves in the stator.
The stator of a helicoidal pump/motor is typically formed by encasing an elastomeric material, which forms the helical grooves, within a cylindrical metal housing. An interference fit is provided between the stator elastomeric form and the rotor for scaling purposes. As a result of is interference fit, the elastomeric form undergoes deformation as the rotor lobes traverse the surfaces of the stator grooves. Thus, the stator must be strong enough to maintain the dimensional stability necessary to ensure a controlled interference fit and durable enough to withstand abrasion from particles in the fluid, yet be sufficiently flexible to deform under the action of the rotor. Consequently, the maximum capability of a helicoidal pump/motor, e.g., the maximum output torque in the case of a motor, is typically limited by the strength of the elastomer.
Unfortunately, the hysteresis associated the repeated cyclic stresses induced by the stator elastic deformation can generate substantial heat. Conventional helicoidal pump/motor stators cannot dissipate heat quickly. Consequently, overheating of the elastomer may result. Over time, such overheating causes deterioration and embrittlement of the elastomer. Such deterioration can lead to failure of the stator, for example, by a phenomenon known as "chunking," in which large pieces of the elastomer are torn off under the action of the rotor. One proposed solution to this problem involves the incorporation of helical tubes within the stator. According to this approach, a portion of the working fluid, typically drilling mud, is diverted so as to flow through the tubes, bypassing the normal flow path and aiding in the transfer of heat from the elastomer. Such an approach is disclosed in U.S. Pat. No. 5,171,139 (Underwood et al.). However, as a result of bypassing a portion of the working fluid, this approach results in decreased performance of the motor. Moreover, if the tubes are narrow, they can become clogged with debris carried along with the working fluid.
Consequently, it would be desirable to provide a stator for a helicoid type pump/motor having improved heat transfer characteristics and increased durability, stiffness and strength.